The invention relates generally to the field of imaging and more specifically, to the field of acousto-optic imaging.
Various imaging techniques have been developed for use in a wide range of applications. For example, in modern healthcare facilities, non-invasive imaging systems are often used for identifying, diagnosing, and treating physical conditions. Medical imaging typically encompasses the different non-invasive techniques to image and visualize the internal structures and/or functional behavior (such as chemical or metabolic activity) of organs and tissues within a patient. Currently, a number of modalities exist for medical diagnostic and imaging systems, each typically operating on different physical principles to generate different types of images and information. These modalities include ultrasound systems, computed tomography (CT) systems, X-ray systems (including both conventional and digital or digitized imaging systems), positron emission tomography (PET) systems, single photon emission computed tomography (SPECT) systems, and magnetic resonance (MR) imaging systems.
Another imaging modality is optical imaging, which operates by propagating light of certain wavelengths through a patient and generating an image based on the propagated light. Different wavelengths of light, including near infrared (NIR) wavelengths in the 700-1000 nm window, may be used to measure optical properties of tissue. Optical properties of tissues may then be used as a contrast mechanism for diagnostic medical imaging (i.e., as a basis for image generation). NIR light is both absorbed and scattered in a turbid medium such as biological tissue. However, scattering greatly dominates absorption in this wavelength range. Light sources and detectors may typically be located at the boundary of the tissue-air interface. The transport of the photon density waves from the source to the detectors and the consequent change in their amplitude and phase can be tracked in tissue with a known distribution of optical properties using a forward model. The inverse problem involves the use of measurements made on the tissue-air interface to reconstruct the spatial map of absorption coefficients in the interior of the tissue. Such values may be used to develop images of the features of internal tissues.
Although conceptually simple, the ill-posed nature and the non-unique solution to the problem of optical reconstruction is further complicated by non-linearity due to scattering, low signals due to absorption, and limited contrast offered by different tissue types. Despite improvements possible with endogenous fluorescence contrast, optically reconstructed images are limited in sensitivity and spatial resolution due to the smearing of endogenous contrast through scattering and absorption. Due to light absorption and scattering by the imaged tissue, diffuse optical imaging typically has relatively poor spatial resolution and anatomical registration. For example, when optical imaging using endogenous contrast is employed for cancer detection, the imaging technique suffers from low or reduced sensitivity and specificity.
Several diffuse optical reconstruction techniques have been formulated and employed in an effort to overcome limitations of diffuse optical imaging, such as those discussed above. Different broad conventional approaches to tackle the complexity of the diffuse optical reconstruction problem are based on linearizing the problem, using multiple wavelengths, measuring the time varying properties of diffusion density waves over ranges of frequencies, detecting ultrasound generated by thermo-acoustic expansion of absorbed light, acoustically modulating the light from the illumination source in situ, employing exogenous fluorescent probes to enhance image contrast, and adding a priori anatomical and/or functional information about the sample from another modality to constrain the reconstruction.
For example, techniques that combine ultrasonic acoustics and light propagation in tissue are promising ways of improving the spatial resolution of diffuse optical imaging. Examples of such techniques include photo-acoustic imaging techniques and acousto-optic imaging techniques. Photo-acoustic imaging techniques typically generate maps of relative optical absorption of a tissue or sample at the wavelengths of excitation light, with strong signals being generated by stronger absorbers. This technique is amenable to generating images of both endogenous and exogenous absorbers. Acousto-optic imaging techniques, by comparison, also generate maps of relative absorption by propagating coherent light into the scattering tissue where the propagated light, through constructive and destructive interference, establishes a speckle field. An acoustic field is applied to the tissue, thereby applying microscopic movement of scattering elements, changing the speckle field in a time-varying manner. These time varying components of the optical signal are inspected and assessments of the spatial distribution of absorption are made. This technique is therefore also amenable to endogenous and exogenous absorption contrast. However, substantial amounts of absorbing material are required to result in a measurable change in the detected signals. As a result, the imaging paradigm is equivalent to measuring a small change in contrast on a largely varying background. The use of exogenous fluorescent probes is advantageous in providing improved sensitivity as one can filter the wavelength of light that is allowed to be extremely selective to fluorescent emission. This is equivalent to measuring a small signal on a flat background. However, fluorescence is typically not a useable mechanism in photo-acoustic imaging, since good fluorescent dyes make poor photo-acoustic dyes, or in acousto-optic imaging, where the incoherent light emitted from fluorescence is not modulated via the same mechanisms as the coherent light and hence does not demonstrate the same interference.
The ill-posed nature of the reconstruction problem may be significantly reduced by a hybrid imaging system that combines the sensitivity of optical imaging (further improved via fluorescence) with the spatial resolution of ultrasound (acoustic imaging). The incoherent light from fluorescence propagating through the scattering and absorbing tissue medium would be modulated during its passage through the focal region of an externally applied focused ultrasound wave. Ultrasonic excitation from an internal or external source may therefore be used to overcome the relatively low resolution of the optical imaging system. However, it has been found that the strength of the modulated signal rapidly deteriorates with increase in depth of the scattering medium (tissue) due to scattering. This therefore places greater requirements on detector sensitivity required to perform deep tissue imaging.
It is therefore desirable to improve the sensitivity in acousto-optic imaging while taking advantage of its improved spatial resolution. It is also desirable to provide improved acousto-optic coupling mechanisms for incoherent light in turbid media that would enable deeper tissue imaging with better sensitivity and resolution. Further, it is desirable to provide acousto-optic imaging system with improved scanning speed and signal-to-noise ratio.